In February you are in real risk of rain—lots of rain, deluge, downpours with the real possibility of lovely sunny days, dress accordingly. Hawaii is subtropical and gets the same storms that hit the West Coast of the Mainland US only a lot warmer. Mid-April offers a much better chance of good weather..
Get a good map (Nelles Maps are good.) and look for roads that have warnings about driving rental cars on. These roads can offer excellent biking. The classic ride is the narrow road on the northeast side leading to Hana. My wife and I road there many years (decades) ago and camped at Seven Sacred Pools. The Pools are now part of the National Park. Beyond the park, the road becomes a rough 4x4 dirt road that circles back to tourist beaches on the south central part of Maui. Looking at the Haleakala NP site http://www.nps.gov/hale/
it looks like some of that road, but not all, is now paved. The south side is the dry side of the island.